Currently, blown films are made predominantly from ethylene polymers. There are references to blowing films of propylene polymers, but none are observed to be commercially successful. Historically it was believed that the low melt strength of propylene polymers inhibited production of blown film with such polymers at commercially feasible rates on standard equipment used for processing polyethylene. Therefore, there has been a desire to increase the melt strength of propylene-based polymers to enhance the ability to manufacture air quenched blown films with such polymers.
The most common method for increasing the melt strength of propylene-based polymers has been to utilize higher molecular weight (Mw) polymers, preferably polymers having melt flow rates less than 2 g/10 min, more preferably less than 1 g/10 min, as determined by ASTM D1238-01 @ a temperature of 230° C. and using a 2.16 kg weight. Another method for increasing the melt strength of propylene-based polymers is to introduce branching into the polymer chains through methods known to one of ordinary skill in the art.
Scheve et al. in U.S. Pat. No. 5,519,785 disclosed the use of polypropylenes having a branching index less than one and having a strain hardening elongational viscosity to blow certain films. The polymers were treated with radiation under specified conditions in a multistep process which involves specialized equipment in steps after polymerization. Such a process is multi step, difficult and preferably avoided. Further, it has been discovered that polypropylene resins containing heavily branched propylene-based polymer chains tend to have reduced optical properties, such as haze and clarity, than non-branched polypropylenes.
Giacobbe and Pufka in U.S. Pat. No. 5,641,848 disclose making blown films from a propylene polymer material of broad molecular weight distribution (MWD of about 4-60), a melt flow rate of about 0.5 to 50 dg/min. and xylene insolubles (at 25° C.) of greater than or equal to 94 percent, said propylene polymer material selected from a broad molecular weight distribution propylene homopolymer and an ethylene propylene rubber impact modified broad molecular weight homopolymer. But this blend forms blown films at rates lower than those used commercially for polyethylene blown films.
The inventor's have surprisingly discovered that propylene-based films can be produced which have an excellent balance of mechanical and physical properties, such as stiffness and optical properties, at commercially acceptable rates using relatively high melt flow rate propylene-based polymers (homopolymers and copolymers) that do not exhibit and/or need to rely upon high melt strength. Further, it has been discovered that these propylene-based resins can be manufactured into both monolayer and coextruded film structures using commercially available air quenched blown film equipment that is typically used in the manufacture of polyethylene-based films.